1. Field of the Invention
The present invention relates to a method for manufacturing semiconductor devices and more particularly to such a method for manufacturing semiconductor devices that includes a step of doping impurities at a high concentration to hemi-spherical grained silicon (hereinafter abbreviated as HSG-Si: Hemi-Spherical Grained Si).
2. Description of the Related Art
There has been a demand recently for even higher integration densities of dynamic random access memories (DRAM's) and other semiconductor devices by increasing the electrostatic capacitance of a unit area of the component capacitor of each memory cell. To meet the demand, such a method has been adopted that increases the electrostatic capacitance by forming in a cylindrical shape either one of the upper and lower electrodes making up each capacitor, for example the lower electrode. According to another method, on the capacitor's surface of the electrode often made of polycrystal silicon are formed HSG-Si to provide irregularities in that surface in order to increase the surface area of the electrode, thus trying to increase the electrostatic capacitance. In this case, however, when HSG-Si is depleted, its electric resistance is increased, thereby preventing the capacitor's capacitance from being sufficiently increased. Usually, therefore, a diffusion method or an ion implantation method is used to dope impurities such as phosphorus into HSG-Si to reduce its electrical resistance.
A technology by means of a diffusion method for doping impurities into HSG-Si grains is disclosed in Japanese Laid-Open Patent Application No. Hei 9-289292. A method, described in this publication, for manufacturing semiconductor devices sequentially comprises steps of: forming inter-layer insulator films on a semiconductor substrate; forming cell contacts; forming polycrystal silicon films having irregularities in the surface; forming an oxidized film of silicon (hereinafter called PSG: Phospho Silicated Glass) on the above-mentioned polycrystal silicon film; diffusing impurities from this PSG film into the irregularly-surfaced polycrystal silicon film; and removing the PSG films. According to this manufacturing method, PSG films must undergo heat treatment at a temperature of about 800-950.degree. C. for about 10-60 minutes.